Background

Cisplatin is a highly effective, broad-spectrum antitumor drug that is commonly used in clinical settings [1] and is effective against a variety of solid tumors [2]. The anticancer mechanism of cisplatin relies on DNA cross-linking, which disrupts DNA replication and transcription factor binding, leading to dysregulated gene expression and cell death [3]. However, clinical application has revealed that cisplatin has serious ototoxic side effects, which typically manifest as bilateral, irreversible sensorineural hearing loss [4] and tinnitus [5]. As the outer hair cells (OHCs) of the cochlear base are vulnerable to these effects, hearing loss first occurs in the high-frequency range [6]. With the accumulation of cisplatin in the cochlear tissue, hearing loss occurs in the middle- and low-frequency ranges and may eventually lead to full-frequency hearing loss [7]. Cisplatin can also damage various structures in the inner ear, including hair cells, the stria vascularis, the basilar membrane, the osseous spiral lamina, spiral ligaments, and spiral ganglion cells [8]. As a consequence, cisplatin-induced ototoxicity causes permanent hearing loss in approximately 60% of adult patients and at least half of pediatric patients [9], [10], [11]. Sodium thiosulfate has recently been approved by the US Food and Drug Administration (FDA) for otoprotection in pediatric patients with local nonmetastatic solid tumors [12]. It only partially reduces high-frequency hearing loss and has no significant protective effect on low-frequency hearing loss. Currently, there are no drugs that inhibit the ototoxicity of cisplatin in adult patients or children with metastatic cancer [13].

Growth arrest and DNA damage-inducible alpha (GADD45A) is involved in stress signaling pathways triggered by physiological and environmental stressors, and participates in multiple cellular processes, including DNA repair, cell-cycle regulation, and stress kinase signaling [14], [15]. During acute stress and inflammation, GADD45A contributes to the chemotactic response of immune cells to lipopolysaccharide (LPS) and other inflammatory stimuli and modulates myeloid-cell responses [16], [17], [18]. Gadd45a deficiency has also been associated with enhanced autophagic activity, characterized by increased microtubule-associated protein 1 light chain 3-II (LC3-II) expression and reduced sequestosome 1 (SQSTM1) levels through modulation of the beclin 1-phosphatidylinositol 3-kinase catalytic subunit type 3 (BECN1-PIK3C3) complex [19], suggesting that the biological functions of GADD45A are context-dependent and may influence both cell survival and cell death. Previous studies have reported that both intense noise [20] and high doses of aspirin [21] significantly upregulate GADD45A expression in the inner ear, suggesting a potential role in hearing loss. Given the protective role of autophagy against cisplatin-induced apoptosis [22], these findings suggest that GADD45A may influence cellular susceptibility to stress-induced injury.

Although the molecular mechanisms underlying cisplatin-induced ototoxicity remain incompletely understood, accumulating evidence suggests that inflammation plays a central role in both processes. Cytokines and chemokines are now recognized as key mediators of cisplatin-induced hearing loss [23], [24]. For instance, after cisplatin treatment, the migration of immune cells expressing CD45, CD68, and ionized calcium-binding adaptor molecule 1 (IBA1) into the cochlea results in the accumulation of the cytokine C-X-C motif chemokine ligand 1 (CXCL1) in spiral ganglion neurons (SGNs) and the organ of Corti in a time-dependent manner. Intratympanic injection of SB225002, a chemical inhibitor targeting the CXCL1 receptor (CXCR2), can protect against cisplatin-induced hair cell loss and hearing impairment [25], [26], [27]. Nuclear factor κB (NF-κB) is a key transcription factor regulating the expression of multiple inflammatory chemokines, including members of the CXCL family. GADD45A mediates p38 phosphorylation and the degradation of inhibitor of nuclear factor κB (IκB), an inhibitor of the NF-κB signaling pathway, leading to NF-κB translocation to the nucleus for the transcription of downstream genes [28]. Moreover, NF-κB1 can stabilize GADD45A through ubiquitination and proteasome-dependent degradation and subsequently activate the mitogen-activated protein kinase kinase 4-c-Jun N-terminal kinase (MKK4-JNK) signaling pathway and induce apoptosis [29]. However, whether GADD45A contributes to cisplatin-induced cochlear inflammation through NF-κB1-dependent inflammatory signaling remains unclear.

Although siGadd45a has shown neuroprotective potential in hippocampal neurons by attenuating neuronal injury and improving functional outcomes [30], the specific regulatory mechanisms of GADD45A in cisplatin-induced ototoxicity and its link to the inflammatory response require further elucidation. The primary objective of this study was to investigate the role of GADD45A in cisplatin-induced ototoxicity and elucidate the underlying molecular mechanisms. Specifically, we sought to elucidate the molecular mechanisms underlying GADD45A-mediated cochlear inflammation and to evaluate its therapeutic potential as a target for preventing cisplatin-induced hearing loss.